Ο λασπωτήρας και το σταντ είναι πάνω στον σκελετό.

Because Greek nouns have grammatical gender.

• ο λασπωτήρας is masculine
• το σταντ is neuter

In the nominative singular, the usual definite articles are:

• ο for masculine
• η for feminine
• το for neuter

So you need to learn each noun together with its article.

Because είναι is both:

• he/she/it is
• they are

So in Greek, the 3rd person singular and the 3rd person plural have the same present-tense form of είμαι.

Examples:

• Ο σκελετός είναι βαρύς. = The frame is heavy.
• Οι τροχοί είναι μεγάλοι. = The wheels are big.

In your sentence, the subject is plural because it is the mudguard and the stand, but the verb form is still είναι.

στον is a contraction of σε + τον.

So:

• σε τον → στον

This is very common in Greek. Similar contractions are:

• σε το → στο
• σε τη(ν) → στη(ν)

So πάνω στον σκελετό literally contains σε τον σκελετό.

Because after σε / στον, Greek uses the accusative case.

The noun changes like this:

• ο σκελετός = nominative
• τον σκελετό = accusative

So:

• Ο σκελετός είναι εδώ. = The frame is here.
• πάνω στον σκελετό = on the frame

This is a very common pattern in Greek: prepositions often require the accusative.

πάνω makes the location more specific.

• στον σκελετό can mean something like in/on/at the frame, depending on context
• πάνω στον σκελετό clearly means on the frame

So πάνω helps express physical position more clearly.

No.

• πάνω σε / πάνω στον usually means on, often with contact
• πάνω από usually means above / over, not necessarily touching

So this sentence means the parts are on the frame, not just above the frame.

Because in Greek, each definite noun normally has its own article.

So:

• ο λασπωτήρας
• το σταντ

This is especially necessary here because the two nouns have different genders. You cannot use one article for both.

So the natural Greek structure is:

• Ο λασπωτήρας και το σταντ...

It is a loanword used in Greek. Greek has many borrowed words, especially for modern objects and technical vocabulary.

Here it is treated as a neuter noun:

• το σταντ

It often means stand, and in bicycle contexts it can mean kickstand.

Also, this noun is commonly used without changing its form, so nominative and accusative are often the same:

• το σταντ
• το σταντ

In this sentence, the articles are the normal choice.

Greek uses the definite article more often than English does. If you are talking about specific parts of a bike, ο λασπωτήρας, το σταντ, and ο σκελετός sound natural.

Leaving the articles out would usually sound more general, less natural, or would change the meaning.

The written accent mark shows the stressed syllable.

• λασπωτήρας → la-spo-TÍ-ras
• είναι → Í-ne
• σκελετό → ske-le-TÓ

σταντ has only one syllable, so stress is not an issue there.

Stress is important in Greek, so it is a good idea to learn each word together with its accent.

Yes. Greek word order is more flexible than English.

The sentence

• Ο λασπωτήρας και το σταντ είναι πάνω στον σκελετό.

is a neutral, natural order.

You could also say:

• Το σταντ και ο λασπωτήρας είναι πάνω στον σκελετό.

The basic meaning stays the same, though the emphasis may shift slightly.

In this sentence:

• ο λασπωτήρας = nominative
• το σταντ = nominative
• τον σκελετό = accusative

Why?

• The first two are the subject of the sentence, so they are in the nominative.
• τον σκελετό comes after σε / στον, so it is in the accusative.

This is a useful sentence because it shows both cases at once.